1. Field of the Invention
The present invention relates to a process for effectively sealing pores in the surface of a molded product, particularly a bonded magnet, and a bonded magnet having pores sealed by this process.
2. Description of the Related Art
A rare earth metal-based permanent magnet such as an Rxe2x80x94Fexe2x80x94B based permanent magnet wherein R is a rare earth metal, represented by an Ndxe2x80x94Fexe2x80x94B based permanent magnet, is used at present in a variety of fields, because it is made using a material abundant in natural resources and inexpensive and having a high magnetic characteristic.
In recent years, in electronic and appliance industries where a rare earth metal-based permanent magnet is used, a reduction in size of a part. has been advanced, and in correspondence to this, there is a need for the reduction in size of the magnet itself and for the complication of the shape of the magnet.
From this viewpoint, attention is paid to a bonded magnet easily formed in any shape, and such a bonded magnet has been already put into practical use in various fields.
The rare earth metal-based permanent magnet contains R which is liable to be oxidized in the atmosphere. Therefore, when the magnet is used without being subjected to a surface treatment, the following problem is arisen: The corrosion is advanced from the surface of the magnet by the influence of a small amount of an acid, an alkali or water to generate a rust in the magnet, resulting in the deterioration and dispersion of the magnetic characteristic. Therefore, it is necessary to form a corrosion-resistant film on the surface of the magnet by an electroplating treatment.
However, for example, when an electroplating is applied directly to the bonded magnet having pores in its surface, a surface detergent and/or a plating solution enters into and remains in the pores, thereby bringing about the corrosion of the magnet.
There is a conventionally proposed process which includes an electroplating treatment carried out after a pore sealing step of impregnating the pores in the surface of the magnet with an inorganic material such as glass or a resin in order to overcome the above-described problem (For example, see Japanese Patent Application Laid-open No.7-201620). When the magnet is immersed into an aqueous solution containing an inorganic component and/or a resin component in the pore sealing treatment, there is a possibility that the magnet is corroded by water, and this method is not desirable. Even when the magnet is immersed into a solution made using a resin itself and a non-water solvent, a curing step is necessarily required after the immersing step. Therefore, this process is not desirable from the viewpoint of the simplification of the producing steps. In the above-described process, it is impossible to impregnate only the pores in the surface of the magnet with an inorganic material and/or a resin and hence, a film layer of the inorganic material and/or the resin is formed on the entire surface of the magnet. The film layer is not formed uniformly due to a sagging. Therefore, even if a surface smoothening treatment is carried out at a subsequent step, an adverse influence is exerted to the surface accuracy of the magnet and as a result, it is difficult to form a plated film having an excellent dimensional accuracy. This film layer may be removed, but an increase in number of producing steps is brought about.
Japanese Patent Application Laid-open No.9-205013 describes a process for sealing pores in the surface of a bonded magnet by throwing blast media and a metal powder simultaneously onto the bonded magnet, or placing the blast media, the metal powder and the bonded magnet into a vessel to treat the magnet by rotating or vibrating the entire vessel. However, this process suffers from a problem that even if the metal powder is once formed into the pores in the surface of the magnet, the metal powder formed into the pores are fallen off or removed by the collision against the contents of the vessel and against the inner wall of the vessel and hence, the sealing of the pores is not achieved sufficiently.
Further, for a ring-shaped bonded magnet suitable to be utilized in various small-sized motors such as a spindle motor and a servomotor used in an actuator, it is necessary to sufficiently seal not only pores in the outer surface (including end faces and so on) but also pores in the inner surface.
Accordingly, it is an object of the present invention to provide a process for sealing pores in the surface of a molded product such as a bonded magnet, which can be carried out selectively and simply in a dry manner for such pores to exhibit an excellent sealing effect, and which cannot exert an influence to the surface accuracy of the molded product.
To achieve the above object, according to a first aspect and feature of the present invention, there is provided a process for sealing pores in a molded product, comprising the steps of placing a molded product having pores in its surface, an inorganic powder, a fat and oil and media into a treating vessel, and supplying a kinetic energy to the contents in the treating vessel, thereby forcing the inorganic powder into the pores and hardening it in the pores.
According to a second aspect and feature of the present invention, in addition to the first feature, the inorganic powder is at least one selected from the group consisting of a metal oxide powder, a metal carbide powder, a metal nitride powder, a metal carbide nitride powder and a metal powder.
According to a third aspect and feature of the present invention, in addition to the second feature, the metal oxide powder is an aluminum oxide powder.
According to a fourth aspect and feature of the present invention, in addition to the second feature, the metal powder is a copper powder.
According to a fifth aspect and feature of the present invention, in addition to the first feature, the media are abrasive stones.
According to a sixth aspect and feature of the present invention, in addition to the fifth feature, the abrasive stone comprises ceramics made by sintering an inorganic powder.
According to a seventh aspect and feature of the present invention, in addition to the first feature, the media are vegetable media.
According to an eighth aspect and feature of the present invention, in addition to the first feature, vegetable media containing a fat and oil are used to place the fat and oil into the treating vessel.
According to a ninth aspect and feature of the present invention, in addition to the first feature, vegetable media having an inorganic powder adhered to its surface by a fat and oil are used to place the inorganic powder and the fat and oil into the treating vessel.
According to a tenth aspect and feature of the present invention, there is provided a process for sealing pores in a molded product, comprising the steps of placing a molded product having pores in its surface and an inorganic powder producing material into a treating vessel, and supplying a kinetic energy to the contents in the treating vessel, thereby forcing an inorganic powder produced from the inorganic powder producing material into the pores and hardening it in the pores.
According to an eleventh aspect and feature of the present invention, in addition to the tenth feature, the inorganic powder producing material is a metal powder producing material for producing a metal powder.
According to a twelfth aspect and feature of the present invention, in addition to the eleventh feature, the metal powder producing material is a copper powder producing material for producing a copper powder.
According to a thirteenth aspect and feature of the present invention, in addition to the eleventh feature, the metal powder producing material is of a needle-like shape and/or a columnar shape having a longer diameter in a range of 0.05 mm to 10 mm.
According to a fourteenth aspect and feature of the present invention, in addition to the tenth feature, the inorganic powder producing material is an abrasive stone comprising ceramics made by sintering an inorganic powder and having a longer diameter in a range of 1 mm to 10 mm.
According to a fifteenth aspect and feature of the present invention, in addition to the tenth feature, a fat and oil is further placed into the treating vessel.
According to a sixteenth aspect and feature of the present invention, in addition to the fifteenth feature, vegetable media containing a fat and oil are used to place the fat and oil into the treating vessel.
According to a seventeenth aspect and feature of the present invention, in addition to the fifteenth feature, an inorganic powder is further placed into the treating vessel.
According to an eighteenth aspect and feature of the present invention, in addition to the seventeenth feature, vegetable media having an inorganic powder adhered to its surface by a fat and oil are used to place the inorganic powder and the fat and oil into the treating vessel.
According to a nineteenth aspect and feature of the present invention, in addition to any of the seventh, eighth, ninth, sixteenth and eighteenth features, the vegetable media are at least one selected from the group consisting of vegetable skin chips, sawdust, chaff, bran, fruit shell and corn cob.
According to a twentieth aspect and feature of the present invention, in addition to the first or tenth feature, the molded product having the pores in its surface is a bonded magnet.
According to a twenty first aspect and feature of the present invention, in addition to the twentieth feature, the bonded magnet is a ring-shaped bonded magnet.
According to a twenty second aspect and feature of the present invention, in addition to the first or tenth feature, the kinetic energy is supplied to the contents of the treating vessel by vibrating and/or agitating the contents of the treating vessel.
According to a twenty third aspect and feature of the present invention, in addition to the twenty second feature, the treating vessel is a treating chamber in a barrel finishing machine.
According to a twenty fourth aspect and feature of the present invention, in addition to the twenty first feature, the ring-shaped bonded magnet is placed into a cylindrical treating vessel, so that the direction of the center axis of the magnet is parallel to the direction of the center axis of the cylindrical treating vessel, and the kinetic energy is supplied to the contents of the cylindrical treating vessel by rotating the cylindrical treating vessel about the center axis thereof.
According to a twenty fifth aspect and feature of the present invention, in addition to the twenty fourth feature, a rod-shaped member is inserted through and disposed in a hole in the ring-shaped bonded magnet, so that it is parallel to the direction of the center axis of the magnet.
According to a twenty sixth aspect and feature of the present invention, there is provided a bonded magnet having pores sealed by a pore sealing process according to the first or tenth feature.
With the process for sealing the pores in the molded product according to the first feature, the inorganic powder is forced into the pores by the action of the media, and the inorganic powder forced into the pores is hardened firmly by the fat and oil. This enables an excellent pore sealing effect to be achieved.
With the process for sealing the pores in the molded product according to the tenth feature, the inorganic powder producing material performs a role of producing an inorganic powder by the collision of pieces of the inorganic powder producing material against one another, against the molded product and against the inner wall of the vessel, and a role as media for forcing the produced inorganic powder into the pores. Thus, an excellent pore sealing effect can be achieved by cooperation of these roles.
The process according to the present invention can be carried out selectively and simply in a dry manner for the pores in the molded product such as the pores in the surface of the bonded magnet to exhibit an excellent pore sealing effect. Then, a corrosion-resistant film such as a plated film having an excellent dimensional accuracy can be formed on the surface of the molded product at a subsequent step without exertion of an influence to the surface accuracy of the molded product.